Antennas with matching sections aside X will not be at 0 where the SWR is at 1:1. Where X=0 is where your antenna is actually resonant. Where the SWR is 1:1 is where the impedance is 50 Ohms. Unless you're using an antenna designed so the feedpoint impedance at resonance is 50 Ohms they'll never align.
Lets take a 1/4 wave antenna as an example. The feedpoint impedance of a 1/4 wave antenna over a perfect ground is 36.8 Ohms. So if you've a perfect ground where X=0 the impedance should show 36.8 Ohms. Now of course this isn't going to show a 1:1 SWR for a CB or ham radio as thats only at 50 Ohms. You will see a 1:1 SWR at some point but X will not be equal to zero where it does.
When I'm doing mobile installs I'll always use a 1/4 wave antenna for testing the RF ground efficiency. I'll not even give a toss what the SWR is. I do not adjust the antenna at all for this, the reason will be explained later. I'm looking for the frequency where X=0 and what the feedpoint impedance is, not for a 1:1 SWR. The further away from 36.8 Ohms it is the poorer the RF ground. As I add more bonding etc I check again. What I expect to find with any improvement is two things:
1) The feedpoint impedance at resonance gets closer to 36.8 Ohms
2) The frequency where X=0 will get lower.
The reason I'm looking for point 2 is that as you improve a RF ground the length of the radiating element shortens for any given frequency (explained in ARRL Antenna Book). By not altering the length of the antenna as you improve the RF ground it means the antenna appears electrically longer so the resononant frequency drops. I continue to add more bonding and doing those two tests until I either hit 36.8 Ohms or the amount the resonant frequency is dropping with every change either stops or becomes very little. At that point you've got it as good as it'll go.
This is also why some people find that with mobile CB antennas they can't seem to tune them. Manufacturers create the lengths assuming a piss poor install because most are so they make the whips quite long. Because the user has got a very good RF ground it means the resonant frequency is below the CB channels but there's not enough adjustment in the antenna to bring it back up without cutting. They think there's something wrong because they don't have an antenna analyser so can only check the SWR on the frequencies their CB can transmit on which often won't be enough. As an example I have very good RF grounding on my mobile install. Sticking my Sirio 5000 on the first time it was resonant at 26MHz. I had to cut almost 3 inches off the whip to bring it back into the CB band. Every CB antenna I install I have to cut.
NanoVNAs are dirt cheap, £40 or so and will save a lot of people a lot of time chasing red herrings and trying to find faults in an installation that don't actually exist but which they think do because of the limitations of the testing they can do with just a transceiver and SWR meter.
